Custom field for registration

[Grouper]

I have a database with email addresses and added a field to my user profile. I would like to on registration or profile save to check if the email address in my custom field is in that database and if so add the user to a usergroup.

I have seen a few tutorials but am not sure where to start with this. I am wanting to do a plugin so if a vBulletin update comes out I don't have to redo my work.

Any help would be appreciated.